1. Field of the Invention
This invention relates in general to a magnetic read sensors, and more particularly to a method and apparatus for providing a ballistic magnetoresistive sensor in a current perpendicular-to-plane mode.
2. Description of Related Art
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads carrying read sensors are then used to read data from the tracks on the disk surfaces.
An MR sensor detects a magnetic field through a change in resistance in its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin dependent scattering, which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
The magnetic moment of the free layer when the sensor is in its quiescent state is preferably perpendicular to the magnetic moment of the pinned layer and parallel to the ABS. This allows for read signal asymmetry upon the occurrence of positive and negative magnetic field incursions of a rotating disk.
Electrical leads and/or shields are positioned to make electrical contact with the ferromagnetic layers. In a CIP (Current-In-Plane) spin valve sensor, the leads are arranged so that electrical current passes through the sensor in a direction that is parallel to the plane of the pinned and free layers. In a CPP (Current Perpendicular To Plane) sensor, the leads are arranged to induce a sense current that passes perpendicularly through the spacer layer from the pinned ferromagnetic layer to the free layer. In either case, when the sense current passes through the sensor, a readback signal is generated which is a function of the resistance changes that result when the magnetic moment of the free layer rotates relative to the pinned layer magnetic moment under the influence of recorded magnetic domains. Resistance is lower when the relative magnetic moments are parallel and higher when the magnetic moments are antiparallel. While CIP (Current-In-Plane) spin valve sensor are a natural configuration for magnetoresistance MR measurements, a CPP (Current Perpendicular To Plane) sensors have proven to pose a difficult technical problem due to lower resistances. Another way in which spin orientation may be used to detect encoded information on a storage medium such as a hard drive is using Ballistic magnetoresistance (BMR). In ballistic magnetoresistance, the sensor size is reduced to just a cluster of ferromagnetic atoms, joined together by two leads. The term “Ballistic” means that the sensor is smaller than the typical scattering path length for the electron. This means that the scattering the electron suffers will be owing to magnetic effects and not to general scattering from atoms in the sensor itself, thereby making the readout process very sensitive. If the electrons flowing in the circuit have been spin-polarized, then when the electrons flow through the sensor, the electrons will scatter more or less to provide greater or lesser resistance, depending on the magnetization state within the atom layer constituting the contact, and on the faint force exerted by the tiny magnetic storage domain being read out by the sensor. However, to develop a BMR sensor, a stable nano-contact region must be provided with the proper conductive and magnetic properties. Further, it would be advantageous to provide a CPP-BMR so that the performance improvements of CPP sensors may be utilized. A CPP-BMR sensor also would also provide further advantages as the sensor is scaled to higher and higher densities (i.e., smaller sensor size) because the signal is inversely proportional to its cross-sectional area.
It can be seen that there is a need for a method and apparatus for providing a ballistic magnetoresistive sensor in a current perpendicular-to-plane mode.